The present invention relates to a liquid crystal display unit and more particularly to a liquid crystal display unit of a passive matrix type including pixels each formed at each intersecting point of scanning electrodes and data electrodes disposed orthogonally to each other and having the transmission factor which is varied in accordance with an average of squared difference between voltages applied to the scanning electrodes and the data electrodes. Further, the present invention relates to a liquid crystal controller capable of driving the liquid crystal display unit of the passive matrix type at a low cost and with high display quality.
Heretofore, a driving frame frequency for obtaining the optimum contrast in an STN liquid crystal is different depending on a response speed of liquid crystal material. It is known that the frequency is 90 to 120 Hz for the response time of 300 ms and 160 to 240 Hz for 100 ms. The frequency is higher as compared with a frame frequency of 60 to 70 Hz used in a CRT or a TFT liquid crystal. For example, in order to convert a signal having the frame frequency into a display signal for the STN liquid crystal, it is necessary that a frame memory for storing display data is used to convert the frame frequency.
On the other hand, in the STN liquid crystal, a driving method of assigning binary information of display on or off to one pixel is used mainly. Accordingly, in order to display gray scale data, that is, data other than the display on or off in one pixel, any special processing is required. As measures for realizing this processing, there is a frame rate control (FRC) system. In the FRC system, several frames are defined as one period and a rate of the display on or off in the period is set to attain the gray scale. Generally, in the FRC system, as shown in FIG. 2, a pattern (hereinafter referred to as FRC pattern) composed of the display on and off in a matrix having a certain size is formed and the FRC pattern is switched for each frame.
As measures for realizing the conversion of the frame frequency and the gray scale processing, there is a liquid crystal controller. The liquid crystal controller performs the frame frequency conversion and the gray scale processing in accordance with a method as shown in FIG. 3 in which the gray scale processing is first performed and then display data are stored in the frame memory to convert the frame frequency or another method as shown in FIG. 4 in which gray scale data are all stored in the frame memory to convert the frame frequency and then the gray scale processing is performed. Such a controller as shown in FIG. 3 is disclosed in, for example, SID '96 Digest, pp. 356 issued by the Society for Information Display and such a controller as shown in FIG. 4 is disclosed in, for example, a data sheet, pp. 98 of a liquid crystal controller 7548 issued by Cirrus Logic Corporation.
In the conventional liquid crystal controller of, for example, the gray scale processing precedent type, the inputted frame frequency of 60 to 75 Hz is used as the switching frequency of the FRC pattern as it is. Accordingly, there is a problem that the switching of the FRC pattern is apt to be seen and recognized. More particularly, it seems that gray scale display portions are moved or flicker. On the other hand, in the frame frequency conversion precedent type liquid crystal controller, since the gray scale processing is performed after the conversion of the frame frequency, the switching frequency of the FRC pattern is the same as the frame frequency of an output of the liquid crystal and is high to some degree. Accordingly, the pattern movement of the gray scale display portions are reduced. However, since it is necessary to store all of the display data including the gray scale information of several bits per pixel into a frame memory, there is a problem that the frame memory capacity increases.